In a microphone, when a static pressure difference occurs between upper and lower spaces of a vibration film, the vibration film is warped due to the static pressure difference, and the sensitivity of the microphone is thus lowered. For this reason, a bent hole that aims to balance the static pressures is sometimes formed between the semiconductor substrate and the vibration film.
However, when even a sound pressure is balanced by the bent hole, the vibration film fails to vibrate by the sound pressure. Therefore, the bent hole is desirably formed as a passage having a high acoustic resistance. The acoustic resistance becomes higher as the cross-sectional area of the passage becomes smaller and the length thereof becomes longer. For this reason, in order to form a bent hole having a high acoustic resistance, the bent hole having a small cross-sectional area and a long passage needs to be formed.
Examples of the microphone formed on a semiconductor substrate include one disclosed in Published Japanese Translation of a PCT Application No. 2004-506394 (Patent Document 1). In this microphone, a bent hole is formed between the semiconductor substrate and the vibration film. In this microphone, however, a cavity is formed below a vibration film by crystal anisotropically etching the semiconductor substrate from the back face side.
Consequently, in this microphone, a slanting face derived from a monocrystalline silicon (111) crystal plane or a crystal plane equivalent thereto appears on the periphery of the cavity, thereby resulting that the opening area of the cavity becomes larger on the back face side of the semiconductor substrate, and the opening area of the cavity is made smaller on the surface side. In this state, the opening area of the cavity on the back face side becomes larger in comparison with the size of the vibration film, thereby making it difficult to manufacture a small-size microphone. Therefore, in the microphone disclosed in Patent Document 1, even when a bent hole having a great acoustic resistance is achieved, it is difficult to manufacture a small-size microphone.
Examples of a method for forming a cavity by carrying out etching on a semiconductor substrate from the surface side include a method for manufacturing a pressure sensor, disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 62-76784 (Patent Document 2). As shown in FIGS. 1(a) to 1(d), this method includes processes in which a sacrifice layer 13 is preliminarily formed between a semiconductor substrate 11 and a diaphragm 12, and the sacrifice layer 13 is isotropically etched from a chemical (etchant) charging port (etching hole) 14 formed to be opened on the diaphragm 12 so that an etching window 15 is formed between the surface of the semiconductor substrate 11 and the diaphragm 12. Thus, the semiconductor substrate 11 is crystal anisotropically etched from this etching window 15 so that a cavity 16 is formed.
However, since, upon microphone manufacturing by using this method, the chemical charging port of the vibration film (diaphragm) is directly connected to the etching window, the acoustic resistance becomes extremely small when this chemical charging port is utilized as a bent hole, and the sensitivity of the vibration film might be thus lowered. Moreover, since the chemical charging port is formed in the center of the vibration film, the strength of the vibration film tends to be lowered, and adverse effects tend to be given to the acoustic characteristic.
Patent Document 1: Published Japanese translation of a PCT application No. 2004-506394
Patent Document 2: JP-A No. 62-76784